📄 struct的对齐 - 陈晓勇的专栏 - csdnblog.htm
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id=_74_108_Open_Text><SPAN style="COLOR: #000000">{<BR><IMG alt=""
src="struct的对齐 - 陈晓勇的专栏 - CSDNBlog.files/InBlock.gif" align=top> </SPAN><SPAN
style="COLOR: #0000ff">char</SPAN><SPAN style="COLOR: #000000"> c;<BR><IMG
alt="" src="struct的对齐 - 陈晓勇的专栏 - CSDNBlog.files/InBlock.gif"
align=top> s1 d;<BR><IMG alt=""
src="struct的对齐 - 陈晓勇的专栏 - CSDNBlog.files/InBlock.gif" align=top> </SPAN><SPAN
style="COLOR: #0000ff"><FONT color=#ff0000>double</FONT></SPAN><SPAN
style="COLOR: #000000"><FONT color=#ff0000> </FONT>e; // long long 类型 vc6
好像没有<BR><IMG alt=""
src="struct的对齐 - 陈晓勇的专栏 - CSDNBlog.files/ExpandedBlockEnd.gif"
align=top> }</SPAN></SPAN><SPAN style="COLOR: #000000">;<BR><IMG alt=""
src="struct的对齐 - 陈晓勇的专栏 - CSDNBlog.files/None.gif" align=top><BR><IMG alt=""
src="struct的对齐 - 陈晓勇的专栏 - CSDNBlog.files/None.gif" align=top></SPAN><SPAN
style="COLOR: #0000ff">#pragma</SPAN><SPAN
style="COLOR: #000000"> pack()</SPAN><SPAN style="COLOR: #000000"><BR><IMG
alt="" src="struct的对齐 - 陈晓勇的专栏 - CSDNBlog.files/None.gif"
align=top></SPAN></DIV></DIV>
<P><BR> 问 <BR> 1.sizeof(s2) = ?<BR> 2.s2的s1中的a后面空了几个字节接着是b?</P>
<P>现在google看到的答案都是<BR><BR> 1.
24<BR> 2. 2</P>
<P>所说的理由如下：<BR> 成员对齐有一个重要的条件,即每个成员分别对齐.即每个成员按自己的方式对齐.<BR>
也就是说上面虽然指定了按8字节对齐,但并不是所有的成员都是以8字节对齐.其对齐的规则是,每个成员按其类型的对齐参数(通常是这个类型的大小)和指定对齐参数(这里是8字节)中较小的一个对齐.并且结构的长度必须为所用过的所有对齐参数的整数倍,不够就补空字节<BR><FONT
color=#0000ff> <FONT
size=3>S1中,成员a是1字节默认按1字节对齐,指定对齐参数为8,这两个值中取1,a按1字节对齐;成员b是4个字节,默认是按4字节对齐,这时就按4字节对齐,所以sizeof(S1)应该为8;<BR> S2中,c和S1中的a一样,按1字节对齐,而d
是个结构,它是8个字节,它按什么对齐呢?对于结构来说,它的默认对齐方式就是它的所有成员使用的对齐参数中最大的一个,S1的就是4.所以,成员d就是按4字节对齐.成员e是8个字节,它是默认按8字节对齐,和指定的一样,所以它对到8字节的边界上,这时,已经使用了12个字节了,所以又添加了4个字节的空,从第16个字节开始放置成员e.这时,长度为24,已经可以被8(成员e按8字节对齐)整除.这样,一共使用了24个字节.</FONT></FONT><BR><BR>-------------------------------------------------------------------------------------------------------------------------------------------------------</P>
<P>这是我测试得到的<BR><BR>答案:</P>
<P>vc 6 下<BR> 1.
24<BR> 2. 2</P>
<P>gcc version 2.95.4 20020320 [FreeBSD] / gcc version 3.4.6 [FreeBSD]
20060305 下<BR> 1.
20<BR> 2. 2</P>
<P>gcc 下struct 变成这样（红色部分），答案两者就一致了。</P>
<DIV
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<DIV><IMG alt="" src="struct的对齐 - 陈晓勇的专栏 - CSDNBlog.files/None.gif"
align=top><SPAN style="COLOR: #0000ff">#pragma</SPAN><SPAN
style="COLOR: #000000"> pack(8)</SPAN><SPAN style="COLOR: #000000"><BR><IMG
alt="" src="struct的对齐 - 陈晓勇的专栏 - CSDNBlog.files/None.gif" align=top><BR><IMG
id=_31_56_Open_Image
onclick="this.style.display='none'; document.getElementById('_31_56_Open_Text').style.display='none'; document.getElementById('_31_56_Closed_Image').style.display='inline'; document.getElementById('_31_56_Closed_Text').style.display='inline';"
alt="" src="struct的对齐 - 陈晓勇的专栏 - CSDNBlog.files/ExpandedBlockStart.gif"
align=top><IMG id=_31_56_Closed_Image style="DISPLAY: none"
onclick="this.style.display='none'; document.getElementById('_31_56_Closed_Text').style.display='none'; document.getElementById('_31_56_Open_Image').style.display='inline'; document.getElementById('_31_56_Open_Text').style.display='inline';"
alt="" src="struct的对齐 - 陈晓勇的专栏 - CSDNBlog.files/ContractedBlock.gif"
align=top> </SPAN><SPAN style="COLOR: #0000ff">struct</SPAN><SPAN
style="COLOR: #000000"> s1...</SPAN><SPAN id=_31_56_Closed_Text
style="BORDER-RIGHT: #808080 1px solid; BORDER-TOP: #808080 1px solid; DISPLAY: none; BORDER-LEFT: #808080 1px solid; BORDER-BOTTOM: #808080 1px solid; BACKGROUND-COLOR: #ffffff">...</SPAN><SPAN
id=_31_56_Open_Text><SPAN style="COLOR: #000000">{<BR><IMG alt=""
src="struct的对齐 - 陈晓勇的专栏 - CSDNBlog.files/InBlock.gif" align=top> </SPAN><SPAN
style="COLOR: #0000ff">short</SPAN><SPAN style="COLOR: #000000"> a;<BR><IMG
alt="" src="struct的对齐 - 陈晓勇的专栏 - CSDNBlog.files/InBlock.gif"
align=top> </SPAN><SPAN style="COLOR: #0000ff">long</SPAN><SPAN
style="COLOR: #000000"> b;<BR><IMG alt=""
src="struct的对齐 - 陈晓勇的专栏 - CSDNBlog.files/ExpandedBlockEnd.gif"
align=top> }</SPAN></SPAN><SPAN style="COLOR: #000000">;<BR><IMG alt=""
src="struct的对齐 - 陈晓勇的专栏 - CSDNBlog.files/None.gif" align=top><BR><IMG
id=_74_108_Open_Image
onclick="this.style.display='none'; document.getElementById('_74_108_Open_Text').style.display='none'; document.getElementById('_74_108_Closed_Image').style.display='inline'; document.getElementById('_74_108_Closed_Text').style.display='inline';"
alt="" src="struct的对齐 - 陈晓勇的专栏 - CSDNBlog.files/ExpandedBlockStart.gif"
align=top><IMG id=_74_108_Closed_Image style="DISPLAY: none"
onclick="this.style.display='none'; document.getElementById('_74_108_Closed_Text').style.display='none'; document.getElementById('_74_108_Open_Image').style.display='inline'; document.getElementById('_74_108_Open_Text').style.display='inline';"
alt="" src="struct的对齐 - 陈晓勇的专栏 - CSDNBlog.files/ContractedBlock.gif"
align=top> </SPAN><SPAN style="COLOR: #0000ff">struct</SPAN><SPAN
style="COLOR: #000000"> s2...</SPAN><SPAN id=_74_108_Closed_Text
style="BORDER-RIGHT: #808080 1px solid; BORDER-TOP: #808080 1px solid; DISPLAY: none; BORDER-LEFT: #808080 1px solid; BORDER-BOTTOM: #808080 1px solid; BACKGROUND-COLOR: #ffffff">...</SPAN><SPAN
id=_74_108_Open_Text><SPAN style="COLOR: #000000">{<BR><IMG alt=""
src="struct的对齐 - 陈晓勇的专栏 - CSDNBlog.files/InBlock.gif" align=top> </SPAN><SPAN
style="COLOR: #0000ff">char</SPAN><SPAN style="COLOR: #000000"> c;<BR><IMG
alt="" src="struct的对齐 - 陈晓勇的专栏 - CSDNBlog.files/InBlock.gif"
align=top> s1 d;<BR><IMG alt=""
src="struct的对齐 - 陈晓勇的专栏 - CSDNBlog.files/InBlock.gif" align=top> </SPAN><SPAN
style="COLOR: #0000ff"><FONT color=#ff0000>double</FONT></SPAN><SPAN
style="COLOR: #000000"><FONT color=#ff0000> </FONT>e;<BR><IMG alt=""
src="struct的对齐 - 陈晓勇的专栏 - CSDNBlog.files/ExpandedBlockEnd.gif"
align=top> } <FONT color=#ff0000>__attribute__ ((aligned
(8)))</FONT></SPAN></SPAN><SPAN style="COLOR: #000000"><FONT
color=#ff0000>;</FONT><BR><IMG alt=""
src="struct的对齐 - 陈晓勇的专栏 - CSDNBlog.files/None.gif" align=top><BR><IMG alt=""
src="struct的对齐 - 陈晓勇的专栏 - CSDNBlog.files/None.gif" align=top></SPAN><SPAN
style="COLOR: #0000ff">#pragma</SPAN><SPAN
style="COLOR: #000000"> pack()</SPAN><SPAN style="COLOR: #000000"><BR><IMG
alt="" src="struct的对齐 - 陈晓勇的专栏 - CSDNBlog.files/None.gif"
align=top></SPAN></DIV></DIV>
<P>可能除了pack，aligned还是有用的。。另
pack中的数字只能为1,2,4,8,16<BR>----------------------------------------------------------------------------------------------------------------------------------------------------</P>
<P>struct
aligned 的目标是提高地址访问的效率，可能导致的问题是在数据类型强制转换时（本质还是内部成员的地址问题）出现的问题。典型的把struct
对象从原端通过网络传递到对端，再还原为struct。这个可能对端的操作平台、编译器等对struct的实际处理是不一致的。。</P>
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